This application claims the priority of Korean Patent Application No. 2003-10383, filed on Feb. 19, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a fuel cell, more particularly, to a direct methanol fuel cell, and even more particularly, to a catalyst for a cathode in a direct methanol fuel cell.
2. Description of the Related Art
Fuel cells are electrochemical devices which generate an electrical energy through electrochemical reaction of fuel and oxygen. Because they are not subjected to the thermodynamic limitations of the Carnot cycle, their theoretical power generating efficiencies are very high. Fuel cells may be used as sources of electric power for industrial, domestic, and automobile driving applications, as well as for electric/electronic products, in particular, portable devices.
Currently known fuel cells are classified into a polymer electrolyte membrane (PEM) type, a phosphoric acid type, a molten carbonate type, and a solid oxide type according to the types of electrolyte used in the cells. If the type of electrolyte is changed, the operation temperature and materials of constitutional elements of a fuel cell are changed.
Fuel cells are also classified into an external reforming type and an internal reforming type according to fuel feeding process. External reforming type fuel cells convert fuel into a hydrogen-rich gas using a fuel reformer before the fuel is delivered to an anode. Internal reforming type fuel cells, also known as direct fuel cells, allow gaseous or liquid fuel to be fed directly into an anode.
A representative example of the direct fuel cells is a direct methanol fuel cell (DMFC). In the DMFC, an aqueous methanol solution or a mixed vapor of methanol and water is mainly fed into an anode. Because the DMFC removes the need for an external reformer and has excellent fuel handling property, it is easier to overcome the problem of miniaturization than other fuel cells.
Electrochemical reactions involved in the DMFC include an anode reaction for oxidizing a fuel and a cathode reaction for reducing a hydrogen ion and oxygen. These reactions are summarized as follows:
Anode reaction: CH3OH+H2O→6H++6e−+CO2 
Cathode reaction: 1.5O2+6H++6e−→3H2O
Overall reaction: CH3OH+1.5O2→2H2O+CO2 
As shown in the above reactions, methanol and water react with each other to produce carbon dioxide, six hydrogen ions, and six electrons at the anode. The generated hydrogen ions travel through a hydrogen ion conducting electrolyte membrane, which is positioned between the anode and the cathode, to the cathode. At the cathode, the hydrogen ions, electrons from an external circuit, and oxygen react to produce water. The overall reaction in the DMFC is to produce water and carbon dioxide by the reaction of methanol and oxygen. Through these reactions, a large portion of energy corresponding to the heat of combustion of methanol is converted to an electrical energy. In order to facilitate these reactions, both the anode and the cathode of the DMFC comprise catalysts.
The hydrogen ion conducting electrolyte membrane acts as a channel through which the hydrogen ions generated by an oxidation reaction at the anode can pass. At the same time, the hydrogen ion conducting electrolyte membrane acts as a separator to separate the anode and the cathode. Generally, the hydrogen ion conducting electrolyte membrane exhibits an ionic conductivity when moisturized with an appropriate amount of water due to its hydrophilicity.
A portion of methanol fed into the anode diffuses into the hydrophilic hydrogen ion conducting electrolyte membrane and then travels to the cathode. This phenomenon is referred to as “methanol cross-over”. A platinum catalyst, which facilitates both the reduction of oxygen and oxidation of methanol, is mainly used for the cathode of the DMFC, and thus methanol delivered to the cathode by the cross-over may undergo oxidation. Such oxidation of methanol at the cathode may significantly lower the performance of the DMFC.
In order to solve this problem, many efforts have been made to develop a hydrogen ion conducting electrolyte membrane capable of preventing permeation of methanol, on one hand, and to develop a catalyst for a cathode having less reactivity with methanol, on the other hand.
By way of an example of the latter, U.S. Pat. No. 6,245,707 discloses a catalyst material comprising transition metal-containing nitrogen chelates.
A currently widely used catalyst for a cathode in the DMFC is platinum particles. In this regard, a catalyst made of a metal alloy may be more advantageous than that made of nitrogen chelates, for good compatibleness with conventional DMFC manufacturing methods.
Examples of a metal alloy catalyst for a cathode in a fuel cell include a Pt—Rh—Fe alloy catalyst as disclosed in U.S. Pat. No. 6,165,635; a Pt—Cr—Cu alloy catalyst as disclosed in U.S. Pat. No. 5,126,216; Pt—Ni—Co and Pt—Cr—Co alloy catalysts as disclosed in U.S. Pat. No. 5,079,107; a Pt—Cu alloy catalyst as disclosed in U.S. Pat. No. 4,716,087; Pt—Cr—Co and Pt—V—Co alloy catalysts as disclosed in U.S. Pat. No. 4,677,092; and a Pt—Co—Cr alloy catalyst as disclosed in U.S. Pat. No. 4,447,506.
However, these alloy catalysts were developed for cathodes in PAFCs, which use phosphoric acid as an electrolyte, to stably facilitate oxygen reduction under an acidic condition. There are no reports about developments of catalysts having less reactivity with methanol.